A 40 GHz, broadband, highly linear amplifier, employing T-coil bandwith extension technique Cheema, H.M.; Mahmoudi, R.; Sanduleanu, M.A.T.; van Roermund, A.H.M. Published in: IEEE Radio Frequency Integrated Circuits Symposium, 008 : RFIC 008 ; June 5-7, 008, Atlanta, Georgia, USA ; [held at] Microwave Week DOI: 0.09/RFIC.008.45650 Published: 0/0/008 Document Version Publisher s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. The final author version and the galley proof are versions of the publication after peer review. The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may down and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Down date: 4. Aug. 08
RTUIF-9 A 40 GHz, Broadband, Highly Linear Amplifier, Employing T-coil Bandwidth Extension Technique Hammad M. Cheema, Reza Mahmoudi, M.A.T. Sanduleanu, Arthur van Roermund Department of Electrical Engineering, Mixed-signal Microelectronics group, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands Philips Research, Prof. Holstlaan 4, 5656A, Eindhoven, The Netherlands Abstract This paper presents a broadband, highly linear amplifier suitable for multi-standard mm-wave applications such as car radar, LMDS and satellite return channel. It can also be utilized as an efficient wideband output buffer, for measurements of mm-wave circuit components. It exhibits a 3-dB bandwidth of 40 GHz with a pass-band gain of 6 db. The presented amplifier is highly linear with an IP3 of +8 dbm. It has been implemented in a bulk 90nm CMOS LP (low power) technology and consumes 3.3 mw from a. V supply. Index Terms CMOS integrated circuits, broadband amplifiers, T-coil, bandwidth enhancement. I. INTRODUCTION The increasing demand of high data rate transmissions and multi-standard multi-mode applications sets stringent requirements for RF transceivers and its components. Broadband amplifiers are vital building blocks of such transceivers and can be used for amplification, as well as for matching and measurement purposes. A broadband amplifier can be an individual building block of an optical transceiver e.g. limiting amplifier, or a part of a building block like the amplification stage of a D- Latch, the gain stage of an oscillator or a multiplexer, etc. The fundamental difference between a broadband amplifier and a tuned or a narrow-band amplifier [] lies in the difficulty to get high gain and large bandwidths at the same time. This is because the gain-bandwidth product is a technology constant depending on the process node used. In order to circumvent this limitation, one possible approach is to distribute gain on many stages []. Although the process limits the gain-bandwidth product of one stage, gain distribution boosts the total gainbandwidth product of the complete amplifier. For a second order roll-off in the amplifier, the bandwidth of the total amplifier is: where BW STAGE is the bandwidth of the individual amplifier and n is the number of stages [3]. The price paid in this approach is usually related to higher on-chip realestate needed and higher power consumption. The main requirement here is to reduce the group-delay distortion of the gain stages by ensuring that peaking at high frequency of the linear blocks is limited. Other techniques include shunt peaking and f T doublers [4] but they are less effective in enhancing the bandwidth of the design. The theoretical bandwidth improvement of the shunt-peaking approach is 70%. However, in practice, the bandwidth extension achievable is only 0 30% [6]. In f T doublers, the input capacitance is reduced by almost a factor, enhancing therefore the input bandwidth of the design. Nevertheless, the output bandwidth is reduced by connecting two devices in parallel at the output. Therefore this approach has also limited value in practice, at millimeter wave frequencies. The design goal for broadband amplifiers is to maximize gain and bandwidth, and, for an acceptable linearity performance, to minimize power consumption. This paper concerns the theoretical approach, the design and the measurement results of a 40GHz broadband amplifier in a baseline CMOS090 LP process. For bandwidth extension, the T-coil technique is employed. Although known from low frequency designs, by taking advantage of the small dimensions of passive components at mm-waves, this design has a bandwidth extension of more than 00% [5]. The paper is organized as follows. Section II presents the bandwidth extension techniques. T-coil design and implementation is discussed in Section III. Section IV introduces some layout aspects and measurement results of the T-coil amplifier are presented in Section V. BW TOTAL 4 / n = BWSTAGE () 978--444-808-4/978--444-809-/08/$5.00 008 IEEE 645 008 IEEE Radio Frequency Integrated Circuits Symposium Authorized licensed use limited to: Eindhoven University of Technology. Downed on May 8, 009 at 09:4 from IEEE Xplore. Restrictions apply.
II. BANDWIDTH EXTENSION TECHNIQUES As mentioned before, inductive peaking and distributed amplification are commonly used techniques for bandwidth extension. It is known that gain of a capacitively ed amplifier rolls off as frequency increases because the capacitor s impedance diminishes. The introduction of an inductor in series with the capacitor generates an impedance which increases with frequency (i.e. it introduces a zero). This nullifies the decrease in impedance of the capacitor and results in a constant impedance level over a broader frequency range as compared to the original RC network [6]. Inductive peaking can be achieved in a number of ways, depending on the placement of the coil. The first type is shunt peaking in which the inductance appears in a branch that is parallel with the capacitance (Fig. ). The second type is a series peaking system in which the inductance is placed in series with the capacitance. A combination of shunt and series peaking is also possible and offers increased bandwidth, than can be achieved by each system alone [6]. IN+ M L shunt R VDD C I L shunt OUT+ C R M OUT- IN- poles. For a general RLC circuit, to obtain constant input impedance at any frequency the relation R = L C must hold. However, this is true, only in a lossless circuit. In practice, owing to losses, the impedance is only constant up to a certain frequency, which, with careful design, can be high enough to work as a broadband amplifier. The relation between different components of the T- coil amplifier are shown below according to [5]. ζ is the damping factor and is chosen as / for maximally flat response in the pass-band (also called Butterworth response). L M is the mutual inductance resulting from the coupling between the two halves of the coil L a and L b. Cb, called the bridging capacitance is used to create parallel resonance and provides further bandwidth improvement. C consists of the output capacitance of the transistors as well as the bond-pad capacitance. Theoretically, the T-coil peaking circuit improves the bandwidth by a factor of.83 as compared to a differential amplifier without inductive peaking. L w a b n = L = = R R C Cb C () () C ζ = 4 Cb (3) C C k Cb = = 6ζ 4 k + (4) R C LM = 4 4 ζ (5) Fig.. Shunt peaking differential amplifier However, the best bandwidth extension method is based on a combination of shunt and double series peaking and is called the T-coil peaking circuit. The circuit schematic of such a network is shown in Fig.. The main characteristic of this circuit are the mutually coupled inductors (also referred as transformer) which form a letter T if the mutual inductance is represented by a separate coil, hence the name T-coil is widely used. The coupling factor k between both halves of the coil (La,Lb or La,Lb) and the bridging capacitance Cb has a certain relationship, depending on the layout of network III. T-COIL DESIGN AND IMPLEMENTATION The T-coil was implemented as an octagonal transformer with an outer ring containing a smaller inner ring as shown in Fig.3. The setup was simulated in Momentum to determine the inductance and coupling coefficient. The portion of the coil from point A to point B forms L a whereas from point B to C form L b. The two rings are perfectly symmetric from all sides implying an equal mutual inductance on both sides of the inner ring. The distance between the outer and inner ring is determined for a specific coupling factor. Metal 6 was used for the coil, however, a small portion of Metal 5 was utilized at the cross-over point. Metal was used as fish bone structure under the coil for substrate isolation. 646 Authorized licensed use limited to: Eindhoven University of Technology. Downed on May 8, 009 at 09:4 from IEEE Xplore. Restrictions apply.
VDD respectively. The chip micrograph is shown in Fig. 4. The active area of the amplifier is 50 x 300 µm. R R L a L a Cb k OUT- OUT+ k Cb L b C C L b IN+ M M IN- Fig. 4. Chip micrograph of the broadband amplifier Fig.. T-coil peaking differential amplifier The inductance L a and L b have a value of 50 ph and a quality factor of 4 at 40 GHz. The best performance of the amplifier is seen for a coupling factor of 0.3 which corresponds to a mutual inductance of 45 ph. The bridging capacitance of 5fF is implemented as a customized metal-to-metal capacitance. The resistance is poly-silicon based, with a value of 30 Ω. I IV. MEASUREMENT RESULTS The broadband amplifier was measured by waferprobing using high frequency differential probes (GSGSG) and 80 hybrids. Agilent E836A PNA vector network analyzer and E4448A PSA spectrum analyzer were used for small-signal and large-signal measurements, respectively. Calibration was performed using Short- Open-Load-Thru (SOLT) standard provided by Cascade Microtech s impedance standard substrate (ISS). The measured small-signal gain (S) and reverse isolation (S) of the amplifier are shown in Fig.5. The -3dB bandwidth of the amplifier is 40 GHz and the maximum gain is 6 db. It is relatively flat over the complete pass-band. The reverse isolation of the amplifier is better than -.5 db. 0 5 0-5 Small-signal gain (S) Reverse isolation (S) Fig. 3. T-coil implementation Power(dBm) -0-5 -0-5 -30-35 -40 IV. LAYOUT AND TECHNOLOGY The amplifier was fabricated in a bulk CMOS 90nm LP technology suitable for low power applications and offers six metallization layers. The transistors in this technology have a measured f T and f max of 07 GHz and 80 GHz Fig. 5. -45 0 5 0 5 0 5 30 35 40 45 50 Frequency (GHz) Gain and reverse isolation of the amplifier 647 Authorized licensed use limited to: Eindhoven University of Technology. Downed on May 8, 009 at 09:4 from IEEE Xplore. Restrictions apply.
The linearity (IP3 and IP) of the amplifier is characterized by carrying out two tone tests. Two sinusoidal tones located at 5 and 6 GHz are applied to the amplifier resulting in an IM3 product at 7(and 4) GHz. The spectrum of the output is shown in Fig.6. The input power of the tones is -0 dbm, resulting in an IP3 of +8 dbm. V. CONCLUSIONS We have presented a broadband amplifier with a 3-dB bandwidth and a pass-band gain of 40 GHz and 6 db, respectively. This amplifier can be used for multi-standard applications operating in the mm-wave frequency range. It can also be employed as a broadband output buffer for high frequency circuit measurements. The amplifier shows excellent linearity performance with an IP3 of +8 dbm and IP of + dbm. ACKNOWLEDGEMENT The authors would like to thank Henry van der Zanden for layout assistance and Philips Research Eindhoven for chip fabrication. Fig. 6. Spectrum for IP3 two tone test IP was measured by injecting two tones at 5 and GHz, generating IM products at 6 (and 4) GHz. The resulting spectrum is depicted in Fig. 7. With an input power of -0 dbm, IP is calculated to be + dbm. The -db compression point for the amplifier is -4 dbm and it consumes.75ma from a.v supply. REFERENCES [] Y. Jin et al., A Millimeter-Wave Power Amplifier with 5dB Power Gain and +8dBm Saturated Output Power, Proceedings of European Solid-State Circuits Conference ESSCIRC 07, Sept.007. [] S.Galal, B. Razavi, 0Gb/s Limiting Amplifier and Laser/Modulator Driver in 0.8µm CMOS Technology, ISSCC Dig. Of Tech. Papers, pp.88-89, Feb. 003. [3] M.A.T. Sanduleanu, Inductor-less, 0Gb/s limiter with 0mV sensitivity and offset/temperature compensation in baseline CMOS8, Proceedings of European Solid-State Circuits Conference ESSCIRC 03, Sept. 003. [4] Hammad M.Cheema et al., A Ka Band, Static, MCML Frequency Divider, in Standard 90nm-CMOS LP for 60 GHz Applications, RFIC Symposium, pp. 54-544, June 007. [5] Peter Stari, Erik Margan, Wideband Amplifiers, Published by Springer, 006. [6] Thomas H. Lee, Planar Microwave Engineering, Published by Cambridge, 004. Fig. 7. Spectrum for IP two tone test 648 Authorized licensed use limited to: Eindhoven University of Technology. Downed on May 8, 009 at 09:4 from IEEE Xplore. Restrictions apply.